Field of the Invention
This invention relates to a positive photoresist developer system and improved method for developing positive photoresist layers, and more particularly, to an improved two-step developer system used in the processing of alkali soluble resin--diazo ketone photoresists to increase the contrast of the developed photoresists. The process of the invention involves the use of a two bath system wherein the substrate coated with positive photoresist is exposed, then immersed in a "predip" bath, rinsed, and then immersed in the second of the two bath system developer baths.
Positive photoresists typically consist of a novolac resin and a diazonaphthaquinone sulfonic acid ester dissolved in an organic solvent. The resist is usually applied by a spin casting technique to silicon wafers which may have a thin coating of silicon dioxide, aluminum, silicon nitride, glass or other material typically used in the fabrication of integrated circuits.
The coated wafers are exposed to light through a mask which provides a pattern for building the various circuit components on the wafer. The pattern is developed by dipping, spraying or placing a puddle of developer solution on the wafer. During spray or puddle development the wafer may be stationary or spun, but excess developer is usually spun off by accelerating the wafer to about 5000 rpm.
The base used in preparing the developer formulation may be selected from the general class of water soluble bases known for use for this purpose and include, for example, metal hydroxides, e.g. sodium hydroxide, potassium hydroxide, sodium silicate, potassium carbonate, and the like.
It is highly desirable that during development there be as little attack as possible on the unexposed resist under conditions which lead to complete removal of exposed resist layer down to the substrate. The lower the rate of dissolution of the unexposed resist relative to the exposed resist, the higher the contrast. High contrast gives a pattern with well defined vertical walls resulting in an accurate reproduction of the mask dimensions in the photoresist coating.
The present invention has application in the "lift-off" operation which is a simplified process during semiconductor manufacture for putting aluminum metal interconnects into place. The undercut resist profile comprises a shadow mask which permits aluminum vapor to be deposited on the substrate in a pattern determined by the developed openings in the resist film. The aluminum deposited on the resist is removed when the resist is dissolved away using a suitable solvent. The remaining aluminum is left firmly bonded to the substrate in a predetermined pattern of interconnects.
The "lift-off" process is described in U.S. Pat. No. 4,212,935. Control of the wall profile, according to the teaching of that patent, is achieved by predipping the resist coated wafer, after exposure but prior to development, into an organic solvent such as chlorobenzene. The use of toxic, combustible organic solvents is undesirable because of health, safety, and environmental concerns. The present invention involves the use of an aqueous solution containing a small amount of fluorochemical or carboxylated surfactant. Such aqueous solutions are preferred, from a health and safety viewpoint to organic solvents.
An unexpected feature of the double dip process using aqueous metal developers is that it permits control of the wall profile from vertical to undercut depending on the exposure; this ability to produce a profile which is broader at the top and narrower at the bottom is particularly important for use in the "lift-off" process.
In co-pending U.S. patent application,, Ser. No. 505,571 filed on June 17, 1983, now abandoned and entitled "High Contrast Photoresist Developer," it is disclosed that the incorporation of a fluorochemical surfactant in an aqueous alkali metal development bath provides high contrast. In that system, which involves a single dip development with alkali metal developers, it has been observed that as the additional exposed wafers are developed, the contrast and sensitivity of the resist tend to deteriorate with prolonged use of the bath. It is thus apparent that a need exists for a safe and efficient system wherein the contrast and sensitivity of the photoresist remains essentially unchanged over the life of the bath.